The present invention relates to a process and an apparatus for determining the position of an object, in particular of a moving object and, more particularly, to such a process and apparatus which utilizes two illumination light beams coming from different directions and aimed at a reflector attached to an object whose position is to be determined so as to be able to detect rotation of the object.
Determining the location of an object by triangulation is generally known, for example, from geodesy. Reference is made to the article in FhG-Berichte 3-88, pages 30-33, only by way of example and, moreover, expressly for the explanation of all terms not made more apparent herein.
A process for determining the location of an object operating on the known principle of triangulation, in which a point of the object is automatically fixed from two measuring heads located at different points with one respective laser beam, is known from the above-mentioned article. The laser beams emitted from each measuring head strike a special reflector attached to the object to be measured and are reflected by this special reflector in parallel to the incidence direction. Insofar as the laser beam does not strike the center of the reflector, it is displaced in parallel to the incident beam. This parallel displacement of the reflected beams is registered by the two measuring heads with position-sensitive diodes (so-called posicons). The output signals of the posicons are applied as an input signal to a digital readjustment control which adjusts the beam orientations via motor-driven rotating mirrors in such a manner that the laser beams "strike" the center of the reflector. Subsequently, the point of intersection of the two beams, i.e. the position of the center of the reflector, is determined by continuously measuring the space angle of the beam with triangulation calculations. In order to do so, the positions of the two measuring heads (i.e. of the fulcrums of the two beams) must be known exactly. The positions of the measuring heads are determined semi-automatically in a reference run when the entire system is set up. In this manner, precise, external measurement of the position of the moving object occurs without contact or retroaction.
With this known process, the translation of the object, respectively of the object coordinate system x', y', z', can be determined in relation to the external coordinate system x, y, z of the permanently installed measuring heads. This prior art process can, for example, be employed to determine "on-line" the location of a robot hand, i.e. the Cartesian coordinates of the point aimed at in space. By virtue of this independent, external measuring system, robots, precision handling and processing machines and the like, in the case of which the usual indirect position measurement via the positions of individual axle drives are falsified or rendered inaccurate by elasticities, play, etc. in the drive elements, can be measured and controlled exactly.
The process known from the above-mentioned FhG-Berichte 3-88 article, pp. 30-33, and the corresponding apparatus, permit very exact determination of the coordinates of a point of, in particular, a moving object relative to the location of the two measuring heads. It is not possible, however, to detect a rotation of the object about the center of the reflector.
Therefore it is an object of the present invention to provide a process and an apparatus which permit, in addition to determining the location of, in particular, a moving object, also detection of rotation of the object.
A feature of the present invention is a partially mirrored reflector. Due to the partial mirroring of the reflector, the beams emitted from the two measuring heads are reflected in parallel to themselves and thereby, the position of the object can be determined by means of triangulation. The two beams pass through the reflector and are detected in the sensor plane. Thus the rotation position of the receiver system can be determined via the point of impact in the sensor plane when the distance of the sensor plane to the center of the reflector is known.
With the present invention, all six degrees of freedom of the receiver system, i.e. of the reflector and the collocated location-sensitive sensors in the system of coordinates x', y', z' collocated to the receiver system, can be determined. As the receiver system in and on the object to be measured is fixed, the object to be measured is also precisely defined as to its location and rotation position.
According to another feature of the present invention, the reflection properties of the reflector permit detecting the difference in position with the aid of position-sensitive sensors so that the light beams and, in particular, the laser beams can always be adjusted to the zero point, i.e. to the center of the reflector, with an adjustment control. With this embodiment, the position can be determined in addition to determining the location.